The present invention relates to a new and improved construction of an infrared radiation-burglary detector -- also referred to in the art as an infrared intrusion detector -- of the type comprising a number of reflector surfaces which focus the radiation arriving from different separate receiving regions upon a common radiation receiver.
With the aid of detectors of this type there is determined the presence of an object, for instance an intruder or a burglar in a protected room or area, by detecting the infrared radiation radiated by such object. The radiation in question can be the inherent radiation of the individual, in a range between 5.mu. and 20.mu., preferably between 7.mu. and 14 .mu.. However, instead of such, there also can be provided a source of infrared radiation and then there is evaluated the radiation reflected by the object or the individual. This is associated with the advantage that there also can be employed radiation in the near infrared range above 1.mu., where most of the optical components, such as lenses and so forth, do not yet exhibit any appreciable infrared absorption. When utilizing the inherent radiation of the individual it is necessary, on the other hand, to carefully select the employed optical components in order to maintain as small as possible the infrared absorption.
The overlapping of the protected room or area by a number of separate viewing fields or receiving regions with intermediately situated dark zones or fields has been found to be particularly advantageous, in order to be able to detect even the slightest movements of an individual who otherwise would not appreciably alter the total radiation. With suitable overlapping of the room, accommodated to the field of application, it is possible to achieve the result that even with slight movements the boundary between a receiving region and a dark zone is exceeded and at the output of the radiation receiver a pulse-like signal or an alternating-current voltage signal appears which can be easily detected by means of a conventional evaluation circuit and utilized for triggering an alarm signal. There have already been proposed different receiving range patterns, for instance the overlapping or subdivision of a room by means of a large number of receiving rays possessing small aperture angle, by receiving strips or circular- or conical-shaped receiving regions.
However, with state-of-the-art arrangements of the aforementioned type the reflector surfaces are arranged such that the different receiving rays or receiving strips intersect in front of or at the front side of the burglary or intrusion detector. This has the advantage that the reflection angle at the individual reflector surfaces can be maintained just slightly smaller than 90.degree.. Thus, even with a relatively poor optically corrected surface there can be obtained good focusing at the radiation receiver and a small aperture angle of the receiving rays or regions. Since such burglary detectors also can be equipped with simple spherical reflectors, attempts have already been made on numerous occasions to utilize such in practice. However, it has been found to be extremely disadvantageous that at the area of the intersecting receiving region, i.e., at the near or close region directly in front of the front side of the detector, the sensitivity is greater by a multiple factor than at the remote region, i.e., at a greater spacing from such front side. Therefore, such devices tend to trigger false alarms when insects or other living organisms are present at their close region. Also, the closure disc which is almost always used with such type devices, and serving to protect or camouflage the device, can result in triggering of a false alarm. This closure disc is generally constructed such that it is only permeable to radiation in the wavelength range of the evaluated radiation, for instance between 7.mu. and 14.mu. . Radiation of a different wavelength is absorbed and heats the closure disc, which consequently again transmits its infrared self-radiation in the direction of the radiation receiver. In the presence of more pronounced disturbance radiation of other wavelength ranges, it is therefore possible for a false alarm to be triggered. Additionally, for this reason it is not possible to completely utilize the sensitivity of such burglary detectors. Prior art burglary detectors of this type are also associated with the drawback that they are extremely susceptible to triggering false alarms and in many instances possess inadequate sensitivity especially at the remote region.